This invention relates to hot melt adhesive compositions which includes a hydrogenated block copolymer such as Kraton G, about 25 to about 250 parts by weight of a hydrocarbon resin of a petroleum or coal tar distillate, aliphatic dienes and mono and diolefins, cyclic olefins of 5 or 6 carbon atoms and hydrogenated polycyclics per 100 parts by weight of the hydrogenated block copolymer, and about 25 to 200 parts by weight of a polybutene or polyiso-butylene per 100 parts of the hydrogenated block copolymer.
Broadly speaking, synthetic adhesives used in packaging can be classified into four categories: water based, solvent based, reactive and hot melt adhesives. Of these four, currently the water based are used most extensively. Usually the water based adhesives are based on emulsion polymers and are applied to porous cellulosic substrates. Energy from the outside in some fashion is applied to the system to evaporate the water in order that a strong bond may be formed. Beside this energy requirement for the formation of the bond, there is another complication with the use of water based adhesive. For a uniform coating a good uniform wetting of the substrate surface is desired, which is not easily achieved.
With the solvent-based adhesives usually a good wetting is achieved; however, their use has been becoming extremely restrictive due to expensive energy requirements for the evaporation of organic solvents, fire hazards associated with the use of these organic solvents and emissions problems with said solvents. The strict government environmental regulations and restrictions concerning worker's exposure to solvent vapors, etc. has placed extra pressure on the packager to use non-solvent based adhesives.
Recently the use of hot melt adhesives has been growing very rapidly in the packaging industry. The hot melt adhesives are generally applied (as the name implies) by conventional extrusion or coating techniques in the temperature range of 250.degree. to 450.degree. F. on one of the surfaces to be bonded. The other surface is brought in contact with the hot surface for a sufficient period of time for the melt to cool, whereupon solidification a strong and durable bond is formed.
The key requirements of resins suitable for hot melt adhesive applications are that they contribute (i) good tackifying characteristics for the polymer, (ii) good physical properties, e.g., good tensile strength at ambient conditions for the formulations, and (iii) reduction in the viscosity at fabrication temperatures.
Certain commercial block copolymers such as Shell's Kraton, Phillip's Solprene and DuPont's EVA copolymers attain the above objectives to a good extent. The adhesives prepared from blends incorporating these polymers have very good adhesive and strength properties at room temperature and can be processed by conventional melt coating and extrusion techniques because of their good flow characteristics. Because of this excellent combination of properties exhibited by ABA type when B represents a polydiene or a polyolefin block and A represents a polystyrene block, the use of Kratons at present in the industry for various pressure sensitive adhesive applications is growing.
However, the conventional block copolymers which are currently being used in adhesives technology, because of their inherent structure, have one serious drawback with respect to their use as a satisfactory adhesive candidate. Most of the conventional Kraton polymers are block copolymers of polystyrene and a polydiene. The polydiene component in Kratons (of industrial interest) is either polybutadiene or polyisoprene. Since both polybutadiene and polyisoprene are highly unsaturated, the Kraton block copolymers comprising either one of these two polymers are highly susceptible to thermal and oxidative degradation. This imposes many constraints on the adhesive users. For example, in order to minimize the degradation presently most often a packaging or a sealant adhesive user has to keep an inert blanket over the adhesive compound not only during formulation and processing which is usually done at somewhat elevated temperatures, but during storage as well. This becomes not only expensive, but at times cumbersome for the packager. Another point of caution experienced with the use of Kratons in adhesives is that the long term end use properties of the final product are highly susceptible to degradation due to UV light.
In order to circumvent these undesired properties of adhesives prepared using Kraton block copolymers, steps to modify the structure of these polymers have been taken. Recently Shell has invented and developed a new generation of Kraton block copolymers in which the mid unsaturated blocks of either polybutadiene or polyisoprene are hydrogenated to yield a saturated mid block. The saturated mid block is stable not only from a processing point of view, but from UV light during storage and use as well. In practice so far it has been found that these new saturated mid block Kraton polymers are difficult to tackify. We have found that certain blends of saturated Kratons when incorporated in certain proportions with selected Escorez resins together with relatively low molecular weight polybutenes such as Amoco's Indopol resins and yield systems which have very good tackifying characteristics. The aggresiveness of tack and other properties of these tertiary blends can be controlled by using carefully selected proportions of these blending ingredients and/or by incorporating certain fillers and plasticizers. All of these blends are mechanically compatible and have good flow properties as judged during milling. They can also be applied from solution if it is deemed necessary for other processing reasons.
The excellent thermal stability inherent in the saturated backbone of Kraton G is a very desirable property for adhesives which will be exposed to high temperatures for long times. Most adhesives based on other unsaturated elastomeric backbones can suffer degradation easily under those conditions.